This invention relates to an inner ear type electroacoustic transducer apparatus or an earphone.
For example an inner ear type electroacoustic transducer apparatus or an earphone shown in FIGS. 8-10 has been already proposed. In this case, a left earphone 1 is shown. With reference to FIG. 10, the ear phone 1 will be explained in detail. In FIG. 10, reference numeral 10 denotes an electroacoustic transducer for converting an audio signal to sound, which is formed in, for example, a moving coil type. Reference numeral 11 denotes a diaphragm and a bobbin 12 around which a voice coil is wound is integrally attached to this diaphragm 11. Then, this voice coil or the bobbin 12 is inserted into a magnetic gap 14 of a magnetic circuit 13. On a sound emanating side of the transducer 10 or front side is provided a protecting plate 2 which opposes the diaphragm 11 and is formed by working a metal plate of substantially the same size as the diaphragm 11. This protecting plate 2 has a plurality of through-holes 21 through which sound waves emitted from the diaphragm 11 are made to pass.
Further, a front side of this protecting plate 2 is covered with an ear piece 3. This ear piece 3 is a portion mounted within a listener's ear or auricle when the earphone 1 is used. To this end, the ear piece 3 has a protruding portion 30 in which a portion corresponding to an entrance of the external auditory meatus is protruded is formed so as to have a substantially the same thickness in any portions and which is made of rubber or plastic material having an appropriate elasticity. Then, a tip end of the protruding portion 30 has a plurality of sound emanating through-holes 31.
A rear face of the element or transducer 10 is covered with a cover 4 formed of plastic material. In this case, an air chamber 41 of a predetermined volume is formed in the back of the element 10 within the cover 4.
The element 10 is connected to an earphone cord 5 through an introducing portion 40 elongated integrally with the cover 4 at an lower part thereof.
The air chamber 41 is open to the outside through a passage hole 42 provided in the introducing portion 40 and thus an acoustic circuit having a predetermined damping resistance is formed in the rear part of the transducer 10.
FIG. 9 shows a state in which the earphone 1 is mounted within an auricle E. With the protruding portion 30 engaged in the external auditory meatus of the auricle E of the left ear, the earphone 1 is worn within a cavum concha F and the introducing portion 40 is introduced to the outside through an inner-tragus notch I between a tragus G and an anti-tragus H. Thus, the electroacoustic transducer 10 or the diaphragm 11 is driven by an audio signal supplied through the cord 5 and a sound is introduced to the external auditory meatus through the sound emanating through-holes 31, so that a man wearing the earphone 1 can hear the sound.
In this case, the cover 4 has provided with a plurality of through-holes 43, which are closed by, for example, acoustic resistant material 44 such as an urethane. Additionally, an acoustic resistant material 16 such as an urethane or the like is charged into a through-hole 15 formed through the center of a plate, a magnet and a yoke constituting the magnetic circuit 13.
However, when a frequency characteristic of the aforementioned earphone 1 is measured, as is indicated for example, by a curve X in FIG. 4, a low acoustic range results thereby producing indistinct sounds.
This reason will be explained by an acoustic equivalent circuit as follows. That is, in the earphone 1, an air chamber 32 is formed in front of the element 10 by the ear piece 3 and this air chamber 32 is substantially closed. Thus, as evident in an acoustic equivalent circuit shown in FIG. 7, the air chamber 32 acts as an acoustic load and is capacitive (capacity CL). Thus, in the low frequency acoustic range, the capacitive control is performed on the damping side so that a substantially flat frequency characteristic is presented. However, in a high frequency acoustic range, an inertial control results, therefore the response diminishes. That is, there is provided a frequency characteristic in which a drop in an acoustic range over 1.5 kHz is conceivable.
Thus, an earphone 1 having a small hole 33 in an ear piece 3 to reduce a level in the low acoustic range as shown in FIG. 11 was produced and its characteristic was measured. FIG. 11 is a sectional view corresponding to FIG. 10 and the same reference numerals are attached to the same components and a description thereof in detail is omitted.
Consequently, an acoustic equivalent circuit thereof is expressed by a parallel circuit formed of the capacity CL of the air chamber 32 and an inertial load ML of the small hole 33 as an acoustic load as shown in FIG. 6. Thus, an inertial load is provided in the low acoustic range, so that as shown by a curve Y in FIG. 4 indicating a result of measurement of frequency characteristic, the level in the low acoustic range is damped steeply thereby eliminating an elongation in a low-pitched sound. Additionally, there occurs a resonance in a middle acoustic range of approximately 1.5 kHz due to the capacitive CL and inertial ML loads. As a result, a sound having a peculiar feature was produced, and therefore it was found that a problem in terms of acoustic quality was unavoidable.